Press the raspberries through a sieve to make a purée. Heat the sugar and measured water in a pan until the sugar dissolves. Bring to the boil and boil for 2 minutes until syrupy. Leave to cool.
Melt the chocolate with 150 ml (¼ pint) of the cream in a bowl over a pan of simmering water (see
Spoon half the cream and syrup mixture into a separate bowl and fold in the raspberry purée. Half-fold the chocolate mixture into the remaining cream and syrup mixture until marbled.
Put alternate spoonfuls of the raspberry and chocolate mixtures in a freezer container. Using a large metal spoon, fold the mixtures together 2 or 3 times until slightly mingled. Freeze overnight until firm.
Transfer the ice cream to the refrigerator about 30 minutes before serving. Serve scooped into bowls.